The present invention relates to a nonvolatile memory which can store information even when the power supply is turned off, and relates to the method of writing such a nonvolatile memory.
There have been proposed various types of conventional nonvolatile memory elements (memory cells) such as MAOS type, FAMOS type, MIOS type and so on. These memories have different structures with regard to each other. For example, to make an electric charge storage structure for nonvolatile storing of logic information in the form of electric charge, an electroconductive film buried in an electrically insulating layer (i.e. floating gate) can be used. Or, multilayers of dielectric materials and ferroelectric film can be utilized. Further, there have been various injection and extraction methods of selectively injecting and extracting electric charge into and from the electric charge storage structure in order to change the charged state of the storage structure. These methods include avalanche injection, tunnel injection, channel injection, and tunnel extraction of electric charge from the electric charge storage structure to change the charged state to the opposite polarity of the extracted electric charge.
The charged state of individual memory cells can be erased concurrently by irradiation of ultraviolet light or X-ray. Further, by a suitable combination of these electric charge injection and extraction methods, one polarity type of electric charge is restored into or extracted from the storage structure which has stored the other polarity type of electric charge to enable electrical erasing and rewriting of memorized contents. These types of memories are called EAROM and E.sup.2 PROM.
A semiconductor region is formed in opposed relation to the electric charge storage structure, or an insulating gate region is electrically coupled to the electric charge storage structure, for use in feeding electric charge to be injected into the electric charge storage structure and for use in receiving electric charge extracted from the storage structure.
Moreover, in practical IC devices, these various types of nonvolatile memory elements (memory cells) are combined with a static RAM cell so as to perform the function of RAM (random access memory ). Very recently, it has been proposed in Japanese Patent Application No. 4635/1987 that a change of the surface potential of the forementioned semiconductor region can be utilized for random-access writing or temporary writing prior to nonvolatile writing.
This random-access writing method is conventionally applied to the nonvolatile memory cell; however, when carrying out random-access reading without effecting nonvolatile writing, the information is automatically erased. Moreover, the smaller the size of the memory cell, the smaller the read signal which causes a serious drawback. Further, according to this conventional method, when the memory cell has a relatively low efficiency of the electric charge capture in the storage structure, the S/N ratio of the signal representative of the nonvolatile memory contents becomes worse, thereby making commercial use difficult.